1. Field of the Invention
The present invention relates to a process for producing polyacrylates having a low content of residual acrylate monomers.
2. Description of the Prior Art
The radically-induced polymerization of .alpha.,.beta.-unsaturated monomers and other monomers which are capable of undergoing radically-induced polymerization is well known. Recent patent applications are mainly concerned with special production processes or special monomer compositions that can be used to obtain defined properties. For example, polymerization processes for producing polyacrylates, which are characterized by significantly improved physical drying properties, are described in EP-A 528,304 and in EP-A 625,529.
Many patent applications describe the production of polyacrylates containing functional groups. For example, EP-A 630,923 describes polyacrylates containing OH groups, which can be combined with polyisocyanates and which result in crosslinked films.
During the radically-induced polymerization of acrylate monomers with peroxide initiators in solution, a feature which cannot be avoided is that a certain portion of the acrylate monomers is not polymerized and remains in the reaction solution. The resulting content of residual monomers varies between 0.5 and 2% by weight.
More stringent environmental standards require that the polyacrylates, which are offered for use in two-component polyurethane coating compositions (PUR) lacquers, have low monomer contents such that during the production of films, substances are not released into the environment which are either toxic or involve pollution of the environment.
An object of the present invention is to develop a process for producing polyacrylates which enables the content of residual acrylate monomers to be reduced to &lt;0.1%, for example.
DD-A 150,212 describes acrylate-maleic anhydride copolymers for use in the production of films, in which the residual maleic acid content is considerably reduced by vinyl ethers or styrene, for example. In DD-A 264,004, the residual monomer content in styrene/acrylonitrile copolymers and in graft polymers on a butadiene rubber is also significantly reduced by the addition of vinyl esters of organic carboxylic acids, such as vinyl acetate or vinyl butyrate.
A disadvantage of both these processes is that after this subsequent treatment the polyacrylate solution contains, instead of the initial monomers, the monomers (vinyl ethers and vinyl acetate) which were added to reduce the initial monomer. Due to their vapor pressure, these added monomers are released to the environment during cure.
Another prior art process is directed to subsequently activating the polymerization batch with peroxide after the completion of polymerization and then of continuing to maintain it at the polymerization temperature for a period time. The monomer content can also be reduced if the polymer solution is distilled after production of the polyacrylate is complete. The residual monomer contents are only reduced to about 0.4% by weight with both of these prior art processes.
Surprisingly, it has now been found that the content of acrylate monomer can be lowered to &lt;0.1% by weight if the reaction solution is mixed at the end of the polymerization with a monomer having a low vapor pressure and a peroxide initiator and subsequently polymerized.